On Mon, 10 May 2004 17:14:06 -0500, "Steve Nosko"
wrote:

|How 'bout this...'
|
|"Jerry Martes" wrote in message
|news | "Richard Clark" wrote in message
| ...
| On Mon, 10 May 2004 06:22:29 GMT, "Jerry Martes"
| wrote:
| making a slotted line.
|
| Hi Jerry,
| [...] As for why a slab style over a cylinder style. That for me is
|taken
| on faith that greater minds, in the person of HP Metrologists, figured
| that one out a long time ago. ...
| 73's
| Richard Clark, KB7QHC
|
| Richard
|
| I used to think the HP slab configuration was choosen so as to put the
| probe in a place of minimum fiels within the line.
| Jerry
|
|
|Gents.
| What you are calling "slab line" is more properly called "Stripline".
|It is a very well defined type of transmission line...that is, it has exact
|equations (unlike the one-sided Micro-strip). I suspect that Jerry's
|conclusion is right-on. At least theoretically, there is zero field along a
|line through the center conductor, parallel to the ground planes. So it
|appears that you would indeed disturb things the least...as long as your
|'probe' exited parallel to the ground planes.
|Ignore everything but the lower right figure on page 3 of the stripline E
|field:
|http://www.altera.com/literature/wp/lvdsboardwp.pdf
|
|It is all sort of 'sucked' into the ground planes...
|The best pix I could find... I think as long as the probe has a small cross
|section when viewed from the line you will disturb the field the least.
|Seems to me in the old days, the probe consisted of a 1N21 diode...

Steve Adam in "Microwave Theory and Applications," pp. 384-385
discusses the H-P slab line slotted section. (No surprise, Adam
worked for H-P) He references, "A New Type of Slotted Line Section",
Proceedings of the I.R.E., Vol. 38, No. 3 (March 1950).

The slab line as shown is a round center conductor between parallel
plates. Equations for this type of line are given in the ITT
"Reference Data For Radio Engineers" and are different from those for
stripline, which uses a flat center conductor.

General Radio manufactured a true coaxial slotted line section
(Discussed in their "Handbook of Coaxial Microwave Measurements"), but
it is much more complicated mechanically than the H-P slab line.

As to disturbing the fields with the probe, of course the fields are
disturbed. Just like every other electrical measurement I can think
of, the device or system under test is *always* disturbed. The
question is just a matter of degree.

As far a building a coaxial slotted line long enough for the frequency
of interest, let me just ask how would you propose to machine a
straight, uniform width slot down the length of a round piece of soft
copper. And then, how would you support the center conductor without
the supports getting in the way of the moving probe. Answer these
questions (to start with) and then you'll understand why H-P used slab
line.